The exemplary embodiment relates to spectrophotometer scanning systems that are suitable for high speed online document color analysis, and finds particular application in connection with the comparison of documents output on different print media.
Spectroscopy involves the measurement and analysis of electromagnetic radiation absorbed, scattered, or emitted by atoms, molecules, or other chemical or physical materials. Each object affects light in its own unique way. When light waves and other incident electromagnetic radiation strike an object, the object's surface absorbs some of the spectrum's energy, while other parts of the spectrum are reflected back from the object. The modified light that is reflected from the object has an entirely new composition of wavelengths. Different surfaces containing various pigments, dyes, and inks (or other chemicals) generate different but unique wavelength compositions. Light can be modified by striking a reflective object such as paper or by passing through a transmissive object such as film or a transparency. The pattern of wavelengths that leaves an object is the object's spectral data, which is often called the “finger print” of the object. Measuring spectral content of the object can give its intrinsic properties. For example, the region of the electromagnetic spectrum visible to the human eye ranges from about 400 nm to 700 nm, and if spectral measurements can be made in that wavelength range, then one can determine the color of the object. The amount of reflectance intensity decomposed at each wavelength is the most complete and accurate description of the color that a person can see. Hence in this case, the spectrophotometer becomes a true color sensor.
Spectrophotometers with a broad range of spectral synthesis have a wide range of applications, including color printing, color measurements in displays, paints, textiles, electronic cameras, chemical analysis, environmental monitoring, measurement of bio-samples for medicine or personal identification, and the like. Commercial spectrophotometers tend to be large in size with many optical elements.
Commercial document scanners commonly sense colors in terms of RGB coordinates, which approximate the human visual system. Most scanners deviate from the human visual system in ways that differ depending on the media and ink being scanned. To address this problem, different characterizations, or profiles can be built for different media. Creation of profiles for multiple media and image combinations can result in a loss of productivity. To address this problem, a wide area scanning spectrophotometer may be embedded in each printing device. For example, full width array (FWA) spectrophotometer systems have been developed which include an array of light sources, such as LEDs, of different colors in the visible range, which are arranged in a multiply repeating pattern. Such systems utilize a linear array of photodetectors to detect an illuminated band of a test target. Measurements obtained on such a device can be used to correct for differences in print media.
A problem with such devices is that they do not always generate measurements which correspond to humanly perceived colors under different types of illumination, such as natural light, fluorescent light, and incandescent light.